In a hallway usability test you grab the next person who passes by and ask them to use the code you just wrote. After hours with your own interface you no longer see its problems; fresh eyes find them in minutes, for free. Spolsky claimed that five people reveal 95% of the usability problems. Jakob Nielsen's article "Why You Only Need to Test with 5 Users", published the same year, gives a more modest figure of about 85%, and recommends several small rounds of testing rather than one large study.
Remote teams have no hallway, so they send links instead. Static hosts such as Netlify 4,096 and Vercel 3,061 publish a preview URL for every pull request (Static Hosting and CDNs), letting a designer, product manager or friendly customer try the change before it merges. Give each tester a task ("buy a medium blue T-shirt"), stay silent, and note where they hesitate.